(1) Field of the Invention
The present invention relates to an image sensor, and more particularly to an image sensor for use in an image reading device such as a facsimile machine and a digital copying machine.
(2) Description of the Related Art
Photoelectric conversion elements used in conventional image reading devices such as facsimile machines may broadly be divided into two kinds, one being a pi-type or a pin-type in which a primary photocurrent is used and the other being an nin-type or a pip-type in which a secondary photocurrent is used. An optical sensor most common is one in which a silicon type material having a lower toxicity than in a halogen material and a high adaptability to processing is used for the photoelectric conversion layer. The photoelectric conversion element using the primary photocurrent is advantageous in that a half tone can be easily accomplished because the current value becomes proportional (.tau..about.1) to the incident light intensity. However, a disadvantage therein is that, since the current value optically obtained is very low or minute, a signal-to-noise (S/N) ratio is lowered due to noise and there is attenuation in signals due to stray capacitance at a wiring intersecting portion. On the other hand, the photoelectric conversion element using the secondary photocurrent is advantageous in that the current value is higher than that obtained in a sensor using the primary photocurrent. However, a disadvantage therein is that it is difficult to detect a half tone because the current value does not easily become proportional (.tau..about.0.6) to the intensity of incident light. Also, such a conventional sensor requires several or some ten milliseconds to respond to a transition from light to dark or from dark to light, and this makes it unsuited to a device such as a G4-facsimile machine which requires a shorter response time than 1 millisecond.
In a sandwiching type sensor using the primary photocurrent, a p.sup.+ -layer (or an n.sup.+ -layer) is formed between an upper electrode and a photoelectric conversion layer. Where the p.sup.+ -type amorphous silicon film layer of 500 Angstroms is formed beneath a transparent electrode, the light that reaches the photoelectric conversion layer is reduced to less than about 70% of the light as compared with that before reaching the p.sup.+ -layer due to the absorption thereof by the p.sup.+ -layer. In order to solve this problem, the p.sup.+ -layer is formed by using amorphous silicon carbide which has a wider bandgap than amorphous silicon.
Also, since the photocurrent obtained by photoelectric conversion is minute, it is necessary to make the current value in the dark state as low as possible in order to obtain a sufficient contrasting ratio between the light and dark. In this relation, there has been attempted to solve the problem by suppressing leakage current at an element facet, which is disclosed in a report under the title "Application of a-Si pin Photodiodes to Image Sensor" by T. Iwabuchi et al., Technical Report of the Institute of Electronics, Information and Communication Engineers, ED84-160, 1985, pp 35-40. In this attempt, the portion of the n.sup.+ - (or p.sup.+ -) layer on the photoelectric conversion layer, the portion protruding from the upper electrode, is etched using the upper electrode as a mask. Also, it is being studied to use individualized elements for purposes of suppressing the leakage current between adjacent picture elements (pixels), or to increase a thickness of the photoelectric conversion layer for purposes of decreasing pinhole defects and leakage current in the photoelectric conversion layer due to the lowering of blocking capabilities.
On the other hand, as disclosed in Japanese Patent Application Kokai Publication Nos. Hei 4-145761 and Hei 3-54959, studies are being conducted on image sensors of the type in which a sensor element and a blocking diode as a switching element are interconnected. A typical example of such an element is shown in schematic sectional view in FIG. 1, in which a glass substrate is designated by the numeral 11, an amorphous silicon film by 12, a p.sup.+ -layer by 13, an n.sup.+ -layer by 14, electrodes by 15, 51 and 52, and a protective film by 16.
In the above image sensor, the photo sensor (left half in the drawings) and the blocking diode (right half therein) are interconnected in a front-to-front form with the upper electrode 15 being used as a common electrode. In this example, the common electrode 15 has a step with a level difference of about 1 micrometer, which leads to cause the occurrence of disconnection in the interconnect, thereby lowering production yields. Also, where both the lower electrodes 52 are used as a common electrode to form a back-to-back connection, there is a high probability for the occurrence of a similar disconnection due to a step difference to develop during the process of providing leads for signals.
Also disclosed in Japanese Patent Application Kokai Publication No. Sho 61-85859 is an image sensor of the type in which a switch by means of a field effect transistor and a photo sensor are formed with a source electrode and an amorphous silicon film layer being commonly used. A typical example of such sensor is shown in schematic sectional view in FIG. 2, in which the same or similar elements as in FIG. 1 are shown in the same reference numerals. The numeral 61 denotes an insulating film and 62 denotes an electrode.
In the image sensor having the above configuration, an n.sup.+ -layer is formed as a contact layer interconnecting the electrode and the amorphous silicon film. This n.sup.+ -layer is formed also beneath the electrode of the diode. Therefore, this image sensor is a photo sensor using the secondary photocurrent and is a sensor in which the response time is long as already explained.
In the conventional image sensors explained above, where the primary photocurrent is used, the image sensor suffers, because it has a sandwiched structure, from disconnection to occur at portions with steps due to the film thickness being larger than about 1 micronmeter, from lowering of through-puts due to a prolonged film formation time, and from absorption of light by the taper-etched step portions and by the p.sup.+ -layer or n.sup.+ -layer disposed at the side at which the light is incident. These problems are also true in the image sensor in which the blocking diodes are connected in series with photo sensors.
In the sensor of the type in which the secondary photocurrent is used, the sensor element is planar but is structured in a nin-type, thus lacking a blocking layer for holes. A response time per picture element (pixel) is several to some ten milliseconds. A problem therefore is that, when it is to be operated at a high speed, the sensor cannot respond to or follow a change of images from white to black and vise versa.